1. Field of the Invention
The present invention relates to a computer system having a function for reproducing and outputting video data recorded on a medium, such as a DVD, using a monitor display. More particularly, it relates to a computer system capable of preventing, for example, copyright video programs from being illegally copied, and a method, employed in the computer system, for controlling the output of reproduced video data.
2. Description of the Related Art
In accordance with recent development of computer and multimedia techniques, so-called multimedia compatible personal computers have been widely put to practical use. Personal computers of this type have a function for reproducing video data (moving picture data) recorded on a medium such as a digital versatile disc (DVD), as well as text data and graphics data.
Video data recorded on a DVD is encoded based on the moving picture experts group (MPEG) phase 2 standard. To reproduce such MPEG-2 video data on a personal computer, an encoded stream of the video data is first read from a DVD drive into the main storage of the personal computer, and then transferred to an MPEG-2 decoder that is realized by software or hardware, under the control of a video reproduction application program. The MPEG-2 decoder performs decoding (decompress) of the coded stream.
The decoded data is once written into a video memory and then displayed on a monitor display, as in the case of standard graphics data provided by an operating system (OS) or an application program. In personal computers equipped with a GUI system, there is a case where a window (overlay surface) is assigned for each application program, and a case where video data is directly written to a video frame memory for display.
In response to strong requests from copyright owners who provide, for example, video software, a number of schemes for preventing illegal copying of video software have been proposed. For example, the Macrovision system developed by the US Macrovision company is the most popular copy protection technique employed in, for example, home VTRs (mainly, for VHS titles). However, no copy protection techniques have yet been established for protecting the video output to the monitor display of a personal computer from copying.
Jpn. Pat. Appln. KOKAI Publication No. 2001-16538, for example, has proposed a structure in which a copy protect function is imparted to a graphics controller, and an interface for executing the Macrovision function is provided in a graphics driver. This publication has also proposed a method in which the Macrovision function is activated for video data whose copying is to be prevented (i.e., where the data contains copy protection information) when the video data is decoded, thereby outputting, to a TV monitor, video data whose copying is to be prevented. This method, however, realizes copy protection for a TV monitor, but does not realize copy protection for a computer display.
In existing video applications, such as DVDs and digital broadcasting, copy protection information is represented by a flag belonging to the MPEG-2 stream. In the field of computers, programs for illegally altering the flag exist. To cope with this, a scheme for superimposing, on video data, copy protection information as digital watermark information is now being developed.
Since conventional personal computers do not have a function for protecting the output to the monitor display from being copied, video data recorded on a DVD and output to the monitor display of a computer is not protected from being copied, regardless of whether the video data contains copy protection information. Copy protection information contained in video data may be illegally copied by software. Further, in conventional personal computers, no consideration is made for the case where a plurality of video reproduction applications have different copy control information items.